The cutting chain of a chain saw as contemplated herein is made up of a repeating sequence of interconnected links. Certain of the links are cutting links and each cutting link has a leading cutting edge. The leading cutting edge is made up of a top plate cutting edge and a side plate cutting edge. These cutting edges intersect to form a bend or corner, and the bend or corner (which can be sharp or rounded) forms a part of the leading cutting edge, i.e. the cutting edge is continuous and has side and top cutting edge portions and a corner cutting edge portion.
The cutting edge portions are simultaneously sharpened, typically with a round file that is drawn across the edges in a precise manner. The configuration of the three cutting edge portions and their relationship is dramatically affected by the manner of sharpening. The chain saw operator's production capability and even safety can be impacted by the cutting edge configuration and a great deal of attention is directed to achieving the desired cutting edge configuration.
One of the most significant features of the cutting edge configuration is the hook angle. The hook angle, at least for the purpose of this description, is considered from a side view of the side plate cutting edge. A reference plane is considered to be established along the chain length through the successive cutting edge corners (at the high point of the corners). A perpendicular line to the reference plane at the corner of the cutter being examined is the base line. A line from the corner of the cutting link through the point on the side plate cutting edge that is 0.020 inches down from the reference plane establishes an angle with the base line that is the hook angle.
The hook angle indicates the sharpness of the top plate cutting edge and establishes the aggressiveness of the cutting link. A cutting link that is too aggressive can overload the chain saw capability and can create a potential safety hazard. A cutting link that is not aggressive enough will cause the chain saw as well as the sawyer to work harder for less production.
Accordingly, there is a desired hook angle. The desired hook angle will not be the same from one type of cutting link to the other, or even for the same cutting link type in different types of wood. Also, a saw chain manufacturer may recommend one hook angle setting whereas a professional sawyer may prefer another. Regardless of what is desirable it is necessary that the various hook angles be identifiable. As the reader will appreciate from the definition of the hook angle given above, it is not a simple task to examine the cutting link and identify from that examination the hook angle of the cutting link.